Common mode rejection ration (CMRR) and common mode input voltage range (CMIVR) are two of many parameters which characterize an operational or differential amplifier. Ideally, a differential amplifier will respond only to differences between two input voltages. However, practical devices also respond to the absolute magnitude of input signals. The CMRR characterizes a differential amplifier's ability to refrain from responding to this absolute magnitude. Furthermore, an ideal differential amplifier will successfully receive two input signals of any absolute magnitude and output a signal which corresponds only to the voltage difference between the two input signals. However, practical devices operate successfully only within a limited range of input signal voltages. The CMIVR characterizes the input voltage range over which a differential amplifier will successfully operate. Obviously, a differential amplifier which has both a large CMRR and a large CMIVR is highly desirable.
In conventional differential amplifiers the CMRR and CMIVR parameters are closely related to each other. Techniques are known for improving the CMRR parameter, but such techniques tend to substantially degrade the CMIVR parameter. For example, conventional differential amplifiers utilize a "nearly-constant" current source which couples to the emitters (or sources) of transistors whose bases (or gates) receive the input signals for the differential amplifier. A "perfectly-constant" current source would exhibit an infinitely high impedance and a corresponding CMRR would be infinitely large. However, practical devices must operate with only a "nearly-constant" current source which has a finite impedance. It is well known that increasing the impedance of this constant current source will correspondingly increase the CMRR. However, conventional techniques for increasing the impedance of this constant current source decrease the CMIVR parameter.
Consequently, a need exists for a differential amplifier design which permits the CMRR parameter and the CMIVR parameter to be independent from one another so that both a high CMRR and a high CMIVR may be achieved.